Fix typo: rename BDSL to BSDL

[djkazunoko]

I think the file name BDSL might be a typo and should be BSDL.

Reasons

• In https://github.com/ruby/erb/blob/master/LICENSE.txt, it says: see the file BSDL
  • The file BDSL was created in the same PR: Clarify the license of erb #53
• In the Ruby core repository, the file name is BSDL: https://github.com/ruby/ruby/blob/master/BSDL

Because of these points, it seems that BDSL may be an unintended typo and the correct name should be BSDL.

Please let me know if this is intentional or if a rename would be appropriate.

Thank you!

Co-authored-by: @blacktoad30 @kaigokan-op